I agree. But they do have a "New! Viking World Voyage I" that departs Ft. Lauderdale and ends in New York. Since it starts and ends at the same latitude, I would consider that one a world cruise.
But we have decided against Viking for another reason. In order to get a cabin that we would be happy with for 180 days, we would have to book a year or more in advance. Anything can happen in that length of time, especially to people of our ages (old!) and I find Viking's cancellation penalty for long cruises unacceptable.